The present invention relates to a copolyester comprising 3-hydroxybutyrate unit (hereinafter referred to as 3HB unit) and one selected from the group consisting of (1) 3-hydroxyvalerate unit (hereinafter referred to as 3HV unit) and 5-hydroxyvalerate unit (hereinafter referred to as 5HV unit), (2) 4-hydroxybutyrate unit (hereinafter referred to as 4HB unit) and (3) 4HB unit and 3HV unit, and also relates to a process for producing the above-mentioned copolyester. More in detail, the present invention relates to a novel copolyester comprising 3HB unit and one selected from the group consisting of (1) 3HV unit and 5HV unit, (2) 4-HB unit and (3) 4HB unit and 3HV unit, being produced by using a microorganism which can accumulate polyester therein, and to a process for producing the novel copolyester.
Since poly-3-hydroxybutyrate (hereinafter referred to as PHB) is a thermoplastic high polymer which is accumulated as the energy-storage substance in the cells of large number of microorganism and shows an excellent biodegradability and an excellent adaptability to living bodies, PHB has attracted one's attention as the "clean" plastic which meets the purpose of environmental protection, and its application to various fields, for example, biomedical material such as suture material and fixation materials for bone fracture and slow-releasing systems which slowly release medicines and agricultural chemicals has been expected for many years. Particularly in recent years, since synthetic plastics have come to present a severe social problems the environmental pollution and the circulation of the resources, PHB has attracted one's attention as a biopolymer which does not depend on petroleum.
However, since PHB is poor in impact strength due to its rigidity, it is unsuitable for practical uses, and from the reason of high cost for producing PHB, its industrial production has not been reduced to practice. Accordingly, in order to improve the impact strength of PHB, a proposal of producing a copolymer of 3-hydroxybutyrate has been raised.
For instance, in Japanese Patent Applications Laid-Open (KOKAI) Nos. 57-150393 (1982) and 59-220192 (1984), a copolymer comprising 3HB unit and 3HV unit has been disclosed. The process disclosed in these Applications is a process for producing the copolyester by propagating the microorganism in the former stage as in the conventional process for producing PHB and culturing the microorganism under the limitation of nitrogen and/or phosphorus in the later stage.
However, in the former Application (No. 57-150393 (1982)), there is a description that a copolyester comprising 99.5 to 50 mol% of 3HB unit and 0.1 to 50 mol% of another ester unit such as 3HV unit can be produced by using propionic acid and isobutyric acid as the substrate in the latter stage of culture. However, in this Application, only a copolyester containing at most 33 mol% of 3HV unit has been shown, and any copolyester containing 3HV unit more than 33 mol% has not been exactly shown.
On the other hand, in the latter Application (No. 59-220192 (1984)), there is a qualitative description that a copolyester comprising at least 40 mol% of 3HB unit and another ester unit is produced by using carbon from the cellular substance of the wasted microorganism after extracting PHB in the latter stage of culture. However, in this Application, no copolyester having an exact ratio of 3HB unit and 3HV unit has been described. Also, the process disclosed therein is complicated, and the composition of the cellular substance varies in the kind and the amount of the component according to the culture conditions. Namely the process is not practical.
Further, in the case where the content of 3HV unit in the copolyester increases from 0 to 33 mol%, it has been known that the nmelting point (Tm) of the copolyester is rapidly lowered from 180.degree. C. to 85.degree. C. [refer to T. L. Bluhm et al., Macromolecules, 19, 2871-2876 (1936)]. This dependency of the melting point on the content of 3HV unit means that it is difficult to industrially obtain product of equal quality.
As a result of the present inventor's studies on industrially profitable and easy method for producing a copolyester in which the molar content of other unit is relatively large compared with the molar content of 3HB unit, it has been found by the present inventor that a copolyester 3 in which the molar content of other units is relatively large compared with the molar content of 3HB unit is formed and accumulated in the microorganism when a microorganism having an ability of producing PHB is cultured in the latter stage, wherein nitrogen and/or phosphorus is limited, in the presence of a carbon source selected from the group consisting of (1) a compound represented by the following formula (I): EQU (CH.sub.2 X.sup.1 CH.sub.2 CH.sub.2 CH.sub.2 COO).sub.n Y.sup.1 (I)
wherein X.sup.1 represents a hydroxyl group or a halogen atom, n represents an integer of 1 to 4 and Y.sup.1 represents a hydrogen atom or a uni- to tetravalent metal atom, (2) a compound represented by the following formula (II); EQU (CH.sub.2 X.sup.2 CH.sub.2 CH.sub.2 COO).sub.n.sup.Y.sup.2 (II)
wherein X.sup.2 represents a hydroxyl group or a halogen atom, n represents an integer of 1 to 4 and Y.sup.2 represents a hydrogen atom or a uni- to tetravalent metal atom, (3) a compound represented by the above mentioned formula (II) and a compound represented by the following formula (III): EQU (CH.sub.2 X.sup.3 CHX.sup.4 CH.sub.2 CH.sub.2 COO).sub.n Y.sup.3 (III)
wherein X.sup.3 represents a hydriogen atom, a halogen atom or a hydroxyl group, X.sup.4 represents a hydrogen atom, a halogen atom, a hydroxyl group or an alkyl group, n represents an integer of 1 to 4 and Y.sup.3 represents a hydrogen atom or uni- to tetravalent metal atom, (4) 1,4-butanediol and (5) .gamma.-butyrolacetone, and on the basis of the finding, the present invention has been attained.